Field of the Invention
This invention relates to an optical writing device being equipped with a plurality of light emitting elements, and to an image forming apparatus being equipped with the optical writing device. The amounts of emitting light of the plurality of light emitting elements change by the self-generated heat. The plurality of light emitting elements are arranged linearly in a main scanning direction.
Description of the Related Art
Organic electroluminescence elements (hereinafter referred to as organic EL elements) are recently used as light emitting elements in displays, lighting equipment, or the like. As seen from FIG. 10, an organic EL element 100 have a structure in which positive electrode 102 composed of a transparent electrode such as indium oxide (ITO) on transparent board 101, at least one organic layer 103 on positive electrode 102, and negative electrode 104 composed of an electrode of aluminum or the like on organic layer 103 are stacked. When drive electrical current flows or drive electrical voltage is applied between positive electrode 102 and negative electrode 104 by electric power supply 105, organic layer 103 emits light. The light is taken out, via transparent electrode 102 and transparent board 101.
As shown by FIG. 11, the amount of light emitted from an organic EL element changes, depending on the element temperature. Organic EL elements generate a great deal of heat by themselves, relative to other light emitting elements (LED (Light Emitting Diode), LD (Laser Diode), or the like). For example, when an organic EL element emits light continuously for about 5 minutes, the temperature of the element rises by 30 degrees Celsius as shown in FIG. 12.
As shown in FIG. 13, when organic EL element emits light by a same drive electrical current, the light emitting amount of the element decreases, as integral light emitting time increases (a light amount deterioration characteristics). The degree of the light amount deterioration is larger, when the element temperature is higher during the drive. Also, the degree of the light amount deterioration is larger, when the electrical current density is larger during the drive.
The above explained organic EL elements may be recently used in an optical writing device of an image forming apparatus. The optical writing device has a plurality of organic EL elements being linearly arranged, parallel to the main scanning direction, and is used when the photo conductor is exposed. However, the photo conductor can not be irradiated with proper amount light, when the amount of light emitting of each element changes by self-generated heat. In consequence, the quality of images being formed by the image forming apparatus may deteriorate.
In case that the amounts of light emitting differ among a plurality of organic EL elements, the ununiformity among temperatures of the elements occurs by self-generated heat. Since the amount of light emitting of each element changes depending on the temperature, the photo conductor can not be irradiated with proper amounts light emitted from elements. In consequence, the quality of images being formed by the image forming apparatus may deteriorate.
According to the above mentioned background, conventional optical writing devices detect temperatures of organic EL elements, and execute temperature correction based on the detection result (see Documents 1 and 2 below).